The present invention relates to a method of preparing borazine modified hydridopolysilazane polymers. The method comprises reacting a hydridopolysilazane polymer with a material comprising at least one borazine ring for a time sufficient to produce the desired polymer. This invention also relates to the novel polymers produced by the above method.
A variety of polysilazane oligomer, cyclics, resins and linear polymers are known in the art. Generally, such polymers are characterized as having backbones with alternating silicon and nitrogen atoms. Cannady, in U.S. Pat. No. 4,540,803, issued Sep. 10, 1985, described a novel class of silazane polymers (hydridopolysilazanes) having a three-dimensional structure formed by the presence of HSi(NH).sub.3 and R.sub.3 SiNH groups. This unique structure afforded such polymers physical properties which are desirable in the formation of various ceramic materials.
Boron-compound-modified polysilazane polymers and various methods for their preparation are also known in the art. For instance, U.S. Pat. No. 5,030,744 granted to Funayama et al. describes the formation of boron-compound-modified polysilazanes by reacting a polysilazane having repeating units comprising: ##STR1## with a boron compound (including a borazine). The polysilazanes described in this reference, however, differ from those claimed in the present invention in that those of the reference are primarily linear or cyclic. In contrast, polysilazanes of the present application are the highly branched, three-dimensional polymers of Cannady, supra. Moreover, the boron compound is used in the reference to crosslink the polysilazanes and thereby increase the molecular weight. On the other hand, the borazine is used in the present invention both to react with the N-Si bond and thus cleave the hydridopolysilazane polymer thereby reducing the molecular weight of the hydridopolysilazane starting material or to react with either the N-H bond or the terminal N-trimethylsilyl bond to add a borazine group to the hydridopolysilazane polymer and thereby essentially maintain the molecular weight of the hydridopolysilazane starting material.
Likewise, European Patent No. 364,323 granted to Ardaud et al., U.S. Pat. No. 4,910,173 granted to Niebylski, and Seyferth et al., J. Am. Ceram. Soc. 73, 2131-2133 (1990), describe various other methods of forming boron-compound-modified polysilazane polymers. These references teach reacting a polysilazane with a trihalogen borane, a boroxine, or a borane, respectively. As with the Funayama patent described above, however, these references do not describe the polysilazanes of the present invention. In addition, neither Ardaud et al. nor Niebylski taught that borazine can be used in the reactions described therein.
Seyferth et al. describe the preparation of a silazane precursor for BN/Si.sub.3 N.sub.4 ceramic materials wherein a diborane is reacted with a mixture of cyclosilazanes obtained by ammonolysis of methyldichlorosilane to form an intermediate, which Seyferth proposed to be silyl-substituted borazine, which in turn is heated and thus polymerized. The present invention can be distinguished from this reference because there is a distinct difference in structure between the one-dimensional cyclosilazanes used therein and the three-dimensional hydridopolysilazanes used in the present invention. Furthermore, Seyferth et al. in Front. Organosilicon Chem., Proc. Int. Symp. Organosilicon Chem., 9th, Meeting Date 1990, 15-27, a later investigation on the same topic, only taught that borazine incorporation retards the crystallization of the Si.sub.3 N.sub.4 ceramic phase after pyrolysis, while the present invention claims that crystallization of both Si.sub.3 N.sub.4 and SiC ceramic phases are retarded in the borazine modified hydridopolysilazane after pyrolysis under a purge of argon to greater 1600.degree. C.
Zank, in U.S. patent application Ser. No 07/810,972, filed Dec. 20, 1991, now U.S. Pat No. 5,169,908 describes a method of preparing boron-compound-modified hydridopolysilazane polymer by reacting borane with the hydridopolysilazane polymer. However, this reference did not teach the reactivity of a cyclic borazine with the highly branched and three-dimensional polysilazane polymer. In addition, the borane modified the hydridopolysilazane precursor by adding a BH.sub.2 group to the hydridopolysilazane polymer, whereas the borazine in the present invention both reacts to cleave the hydridopolysilazane precursor and to add the borazinyl group to the polymer.
Therefore, what has not been described in the prior art is the use of borazines to modify the specific polymers claimed herein. The present inventors have discovered that such derivatized polymers have the same or lower molecular weight than the polysilazane precursors and have properties superior to those known in the art.